Change best effort queue behaviour in dispute-coordinator (#6275)

* Change best effort queue behaviour in `dispute-coordinator`

Use the same type of queue (`BTreeMap<CandidateComparator,
ParticipationRequest>`) for best effort and priority in
`dispute-coordinator`.

Rework `CandidateComparator` to handle unavailable parent
block numbers.

Best effort queue will order disputes the same way as priority does - by
parent's block height. Disputes on candidates for which the parent's
block number can't be obtained will be treated with the lowest priority.

* Fix tests: Handle `ChainApiMessage::BlockNumber` in `handle_sync_queries`

* Some tests are deadlocking on sending messages via overseer so change `SingleItemSink`to `mpsc::Sender` with a buffer of 1

* Fix a race in test after adding a buffered queue for overseer messages

* Fix the rest of the tests

* Guide update - best-effort queue

* Guide update: clarification about spam votes

* Fix tests in `availability-distribution`

* Update comments

* Add `make_buffered_subsystem_context` in `subsystem-test-helpers`

* Code review feedback

* Code review feedback

* Code review feedback

* Don't add best effort candidate if it is already in priority queue

* Remove an old comment

* Fix insert in best_effort

polkadot/node/subsystem-test-helpers/src/lib.rs

@@ -177,7 +177,7 @@ where
 /// A test subsystem context.
 pub struct TestSubsystemContext<M, S> {
 	tx: TestSubsystemSender,
-	rx: SingleItemStream<FromOrchestra<M>>,
+	rx: mpsc::Receiver<FromOrchestra<M>>,
 	spawn: S,
 }
 
@@ -239,7 +239,7 @@ pub struct TestSubsystemContextHandle<M> {
 	///
 	/// Useful for shared ownership situations (one can have multiple senders, but only one
 	/// receiver.
-	pub tx: SingleItemSink<FromOrchestra<M>>,
+	pub tx: mpsc::Sender<FromOrchestra<M>>,
 
 	/// Direct access to the receiver.
 	pub rx: mpsc::UnboundedReceiver<AllMessages>,
@@ -280,11 +280,22 @@ impl<M> TestSubsystemContextHandle<M> {
 	}
 }
 
-/// Make a test subsystem context.
+/// Make a test subsystem context with `buffer_size == 0`. This is used by most
+/// of the tests.
 pub fn make_subsystem_context<M, S>(
 	spawner: S,
 ) -> (TestSubsystemContext<M, SpawnGlue<S>>, TestSubsystemContextHandle<M>) {
-	let (overseer_tx, overseer_rx) = single_item_sink();
+	make_buffered_subsystem_context(spawner, 0)
+}
+
+/// Make a test subsystem context with buffered overseer channel. Some tests (e.g.
+/// `dispute-coordinator`) create too many parallel operations and deadlock unless
+/// the channel is buffered. Usually `buffer_size=1` is enough.
+pub fn make_buffered_subsystem_context<M, S>(
+	spawner: S,
+	buffer_size: usize,
+) -> (TestSubsystemContext<M, SpawnGlue<S>>, TestSubsystemContextHandle<M>) {
+	let (overseer_tx, overseer_rx) = mpsc::channel(buffer_size);
 	let (all_messages_tx, all_messages_rx) = mpsc::unbounded();
 
 	(